Electrophotography can be used to create photographic quality multicolor toner images when the toner particles are small, that is, less than about 10 micrometers, and the receivers, typically papers, are smooth. Electrophotography typically involves the steps of charging a photoconductive element, exposing the photoconductive or dielectric element to create an electrostatic latent image, toning the electrostatic image, transferring the toner to a receiver, and fixing the toner to a receiver. A typical method of making a multicolor toner image involves trichromatic color synthesis by subtractive color formation. In such synthesis, successive latent electrostatic images are formed on an element, each representing a different color, and each image is developed with a toner of a different color. Typically, the colors will correspond to each of the three primary colors (cyan, magenta and yellow) and black, if desired. The electrostatic images for each of the colors can be made successively on a photoconductive element by using filters for each color separation to reflect only the light corresponding to each color in the image to the photoconductive element. After developing each color separation, it can be transferred from the photoconductive element successively in registration with the other color toner images to an intermediate transfer member and then all the color toner images can be transferred in one step from the intermediate transfer member to a receiver. After all the color toners have been transferred to the receiver, the toners are fixed or fused to the receiver. To match the photographic quality produced using silver halide technology, it is preferred that these multicolor toner images have high gloss.
Two types of fuser systems have been used for applying heat and pressure to fuse and fix the toner particles to the receiver, namely, fuser roller systems and fuser belt systems. A problem with fuser roller systems has been that the release temperature of the rollers, that is, the temperature at which the receiver sheet leaves the nip of the rollers, is high. The toner then acts as a hot melt adhesive and can cause the receiver sheet to adhere to the roller. One way to improve the release of the toner and receiver from the fuser roller is to apply a silicone release oil to the roller. Release oils have, however, several disadvantages. Some of the release oil can remain with the fused image sheet and give the sheet an oily feel. It is also difficult to write on a sheet that has release oil on its surface and, when the sheet is handled, fingerprints are readily seen. Release oils also can coat the inside of the electrostatographic machine and may affect the machine reliability. Further, the mechanical complexity of the oil delivery system affects the reliability of the machine.
To avoid the use of release oils, it is known to add low molecular weight polyolefins or functionalized fatty waxes to toner compositions to improve the release of toner from fuser rollers. These additives help provide release from the roller surface if the roller has low surface energy. The low molecular weight polyolefins or functionalized fatty waxes, however, tend to coat the surface of the fuser roller, leading to roller failure. It is also difficult to form images having high gloss with fuser rollers.
The above-mentioned problems encountered with fuser rollers can be overcome by using the alternative system--namely, fuser belts. The concept of fuser belts is disclosed, for example, in U.S. Pat. No. 5,089,363 to Rimai et al. The background art discloses several broad classes of materials useful for fuser belts. for example, U.S. Pat. No. 5,089,363 discloses that metal belts coated with highly crosslinked polysiloxanes provide fused toner images having high gloss. Such polymeric release coatings, however, have poor adhesion to the usual belt substrate materials. Also, the coatings wear rapidly when they contact an abrasive surface such as bond paper or uncoated laser print paper under heat and pressure for repeated cycles. U.S. patent application Ser. No. 08/812,370, filed Mar. 5, 1997 discloses that seamless polyimide resin belt having an intermediate layer of a highly crosslinked silicon resin and a surface layer of a silsesquioxane polymer can produce fused toner images of high gloss and has good release properties without the use of a release oil. However, having an intermediate layer increases the fuser belt cost and complicates the manufacturing process.
There is a need for a fuser belt that can form a fused toner image of high gloss, that is also durable and that readily releases toner images without requiring a silicone or other type of release oil. It is also desirable that such an overcoat be a single layer and made with cost-effective materials.